I've recently tried two black liquid eyeliners in an attempt to replace the Stila liner (reviewed here) that ended up getting so frayed it's now unusable. Normally I don't like to have two products open at the same time that perform essentially the same function (much less three), but I disliked the first replacement liner I tried so much that I bought another. So here's a spoiler: the winner of the showdown is Liner #2, the absurdly named Physician's Formula Eye Booster 2-in-1 Lash Boosting Eyeliner + Serum in Ultra Black. I'm going to review the drugstore eyeliner that I do not recommend first (L'Oréal Lineur Intense Felt Tip Liquid Eyeliner in Carbon Black), and the much more impressive PF Lash Booster (hereafter) second.
First, let's look at swatches of both of them, with Stila's eyeliner as a comparison. On the left, I drew a single line to show the initial opacity and thinness of the point, and then I went back over the same swatch several time to show what happens when you add multiple layers.
Left to right: PF Lash Booster in Ultra Black, L'Oréal Lineur Intense in Carbon Black, and Stila Stay All Day Liquid Liner in Intense Black.
The Stila liner is more completely black on the first swipe than either of the other two and also has more sheen. Because of its frayed tip, that's as fine a line as I can get (and it's impossible to use on my eyelid, because there are fibers pointing in all directions). The PF Lash Booster is a close second, but you can see in the sheerer areas at the edges of the swatches that it's a warmer black than the Stila. It's also more matte, which can make it look darker on the eyelid. When you swatch all three of these eyeliners next to one another, the L'Oréal swatch in the middle looks almost brown. It's also much sheerer, even on the right where I tried to build it up.